As with photographic film cameras, compactness, high quality images, and low distortion are required for lenses used in digital and video cameras. In addition, the use of a solid-state image pickup element, such as a CCD array, in digital and video cameras requires that the imaging optical systems of such cameras satisfy particular conditions.
One of the traditional requirements for a solid-state image pickup element, such as one using a CCD array, to most effectively detect light is that the image pickup plane of the element be nearly perpendicular to the central ray of the light being focused on the image pickup plane at all image heights. That requires that the exit pupil position be at a substantial distance (hereinafter termed the exit pupil distance) from the image pickup plane. For further compactness, recently developed CCD arrays receive light most effectively when the exit pupil is located at a finite distance on the object side of the image pickup plane. Reducing the required exit pupil distance is a current subject of development.
Auto-focus systems are commonly used in digital and video cameras, and a high speed of focusing is generally desired, if not demanded. Inner-focus lens systems and rear-focus lens systems, which allow reduced lens weight and shorter lenses, are compatible with auto-focusing and high speed focusing, and are widely used for zoom lenses. As for the number of lens groups, a three-group configuration is preferable to a two-group configuration because it may also be relatively compact and provide better correction of aberrations.
An example of a three-group zoom lens is disclosed in Japanese Laid-Open Patent Application 2000-284177. This application discloses a three-group zoom lens suitable for use with CCD arrays that receive light most effectively when the exit pupil is located at a finite distance. The three-group zoom lens takes advantage of these recently developed CCD arrays and has a small distance between the second and third lens groups when the zoom lens is focused at infinity, which assists in achieving compactness of the zoom lens.
The three-group zoom lens described in the application above is configured suitably for CCD arrays that have an optimum exit pupil distance of approximately five times the maximum image height. However, it receives light less effectively with CCD arrays that have smaller optimum exit pupil distances. Recently developed CCD arrays have an optimum exit pupil distance of approximately 3.5 to five times the maximum image height for use in even more compact camera optical systems. There is a demand for compact three-group zoom lenses designed for use with such CCD arrays.
The three-group zoom lens configuration disclosed in the application above satisfies the requirement of maintaining a constant distance between the second and third lens groups when the zoom lens is focused at infinity but the third lens groups consists of three lens components. There is a demand for three-group zoom lenses that are even more compact but still provide excellent optical performance.
The present invention is a three-group zoom lens suitable for use with solid-state image pickup devices, such as CCD arrays, that are used in digital and video cameras. The three-group zoom lens of the present invention has an optimum exit pupil distance of approximately 3.5 to five times the maximum image height, a zoom ratio of approximately two, a small size, and its aberrations are favorably corrected.